Re: [PATCH] mm, page_alloc: fix build_zonerefs_node()

[Juergen Gross]

On 07.04.22 13:07, Michal Hocko wrote:
> On Thu 07-04-22 12:45:41, Juergen Gross wrote:
> > On 07.04.22 12:34, Michal Hocko wrote:
> > > Ccing Mel
> > >
> > > On Thu 07-04-22 11:32:21, Juergen Gross wrote:
> > > > Since commit 9d3be21bf9c0 ("mm, page_alloc: simplify zonelist
> > > > initialization") only zones with free memory are included in a built
> > > > zonelist. This is problematic when e.g. all memory of a zone has been
> > > > ballooned out.
> > >
> > > What is the actual problem there?
> >
> > When running as Xen guest new hotplugged memory will not be onlined
> > automatically, but only on special request. This is done in order to
> > support adding e.g. the possibility to use another GB of memory, while
> > adding only a part of that memory initially.
> >
> > In case adding that memory is populating a new zone, the page allocator
> > won't be able to use this memory when it is onlined, as the zone wasn't
> > added to the zonelist, due to managed_zone() returning 0.
>
> How is that memory onlined? Because "regular" onlining (online_pages())
> does rebuild zonelists if their zone hasn't been populated before.

The Xen balloon driver has an own callback for onlining pages. The pages
are just added to the ballooned-out page list without handing them to the
allocator. This is done only when the guest is ballooned up.

So the problem is that a new zone is being populated, but it won't have
free pages when the zonelists are rebuilt.


Juergen

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